Most industrial processes produce wastewater which must be treated to remove contaminants prior to discharge into surrounding rivers, lakes and streams. Several physical, chemical and biological processes exist for treating and controlling the level of various contaminants in wastewater effluents. One of the most common methods for removing biochemical oxygen demand (BOD), a common expression for the degree of organic pollutants in the wastewater, is the aerobic activated sludge process. In this process, untreated wastewater is introduced into a suspension of microorganisms, primarily bacteria, where the BOD is metabolically oxidized. After a sufficient retention time under aerobic conditions, the wastewater enters a clarifier or sedimentation basin where the biomass separates as settled sludge from the liquid and the treated wastewater is discharged into receiving waters.
In order to remove organic pollutants from the wastewater efficiently, microorganisms (bacteria) must have a growth environment with adequate nutrients. Nutrients are chemical elements required by a cell (microorganism) for it to live and reproduce. Major nutrients necessary for the synthesis of new cells are carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, magnesium, calcium, iron, sodium and chlorine. However, industrial wastewaters, especially those generated by pulp and paper mills, often lack sufficient nitrogen and phosphorus to promote cell synthesis. For this reason, nitrogen and phosphorus are frequently referred to as the essential or limiting nutrients for biological wastewater treatment processes.
Accordingly, industry typically controls nitrogen and phosphorus levels in biologically treated wastewater to promote growth of microorganisms which breakdown organic pollutants. Anhydrous ammonia (NH.sub.3) or aqua ammonium (NH.sub.4 OH) are the most common nitrogen supplements used as they are the most economical and readily usable forms of nitrogen. When anhydrous ammonia or aqua ammonia is used as a nitrogen supplement, wastewater operators generally monitor the effluent ammonia concentration as a control parameter for ammonia addition. To ensure adequate ammonia in the wastewater aeration tank most operators keep the ammonia effluent concentration at 1.5 mg/L or higher. However, because the ammonia concentration of the effluent does not reflect the amount of nitrogen available from organic nitrogenous compounds, nitrate (NO.sub.3) or nitrite (NO.sub.2) contained in the wastewater, monitoring only the ammonia level often results in excess nitrogen addition to the wastewater. Excess nitrogen in the discharged wastewater may lead to excessive plant growth and algae blooms which cause bad taste, bad odor, dissolved oxygen (DO) depletion, and fish kills. Because of the eutrophication of our lakes and rivers there is a tremendous demand for nutrient control in the wastewater treatment plants throughout our land.
To prevent excess addition of nitrogen into the wastewater, operators should monitor the Kjeldahl Nitrogen (TKN) concentration of the wastewater and utilize the TKN as a control parameter for ammonia addition. TKN represents the total amount of ammonia-N and organic-N contained in the wastewater. Although utilizing TKN as the control parameter reduces excess nitrogen in the discharged water, the TKN test is very time consuming and difficult for wastewater operators to run. Therefore, the TKN is rarely used by wastewater operators as a control parameter for ammonia addition.
Supplemental phosphorus is typically added to the wastewater by charging phosphoric acid. Like the nitrogen addition, wastewater operators often add excess phosphoric acid to the wastewater to ensure adequate phosphorus for biosynthesis. Phosphoric acid is the preferred phosphorus supplement because most organisms can assimilate and directly use ortho-phosphate (ortho-p) for cell growth. Typically, wastewater operators maintain a minimum ortho-p concentration in the effluent of 0.5 mg/L to ensure that the phosphorus requirements of the microorganisms are satisfied.
Paper mills sometimes use ammonium poly-phosphates (liquid fertilizers) as a source of phosphorus for wastewater treatment. However, poly-phosphates must be converted to ortho-phosphates before they can be used in biosynthesis. Accordingly, use of poly-phosphates often results in excessive phosphorus in the final effluent due to the slow conversion of poly-phosphates to ortho-phosphates. This slow conversion makes controlling the ortho-phosphate concentration level of the effluent difficult often resulting in over charges of poly-phosphates and excess phosphorus in the effluent.
In addition to the problems associated with the excessive addition of nitrogen and phosphorus caused by current industry practices and methods, wastewater treatment operators are plagued by other inefficiencies. Most notably, anhydrous ammonia, while an adequate nitrogen source, has many operational disadvantages. For example, air emissions of ammonia are significant, especially in intensively mixed and aerated wastewater systems, as ammonia vaporizes when added to wastewater in excess. Further, the presence of excess ammonia in the wastewater system promotes nitrification in the aeration system as ammonia is oxidized and converted to nitrate. This nitrification exerts an extra oxygen demand on the wastewater system. Still further, excessive nitrification leads to high levels of nitrate in the secondary clarifier and settling pond which leads to denitrification, a process which generates gas and adversely affects the sludge settling process. In addition, the industry incurs substantial costs associated in complying with increasingly oppressive health and environmental regulations targeting the use and storage of anhydrous ammonia.
The present invention eliminates the use of ammonia, provides an efficient means for supplementing nitrogen and phosphorus, and ensures adequate availability of nutrients for sustained biosynthesis of microorganisms which promote the breakdown of organic pollutants while minimizing the discharge of excess eutrophication causing nutrients common in existing wastewater treatment operations.